This invention is particularly applicable to inductively heating and quench hardening the cylindrical toothed surface of an axially elongated, helically toothed gear and will be described with particular reference thereto. However, the invention has broader applications and may be used for quench hardening other elongated workpieces with significantly textured cylindrical surfaces generally concentric with a central axis.
It is desirable to harden the toothed surface of a gear to enable that surface to withstand the wear and contact forces exerted during operation of a high power transmitting gear train. The surfaces to be hardened are those which intermesh with other gears including the connecting surfaces between the gear teeth as well as the flanks and tips of the gear teeth themselves. It is desirable to keep the body of the gear and gear teeth beneath the hardened surface relatively soft to provide strength and ductility to the gear structure. Ideally, the hardened gear has a hardness pattern extending to a uniform and shallow depth across the entire hardened surface to provide the resistance to surface abrasion associated with hardening while at the same time preserving the strength of the underlying material by avoiding the brittleness associated with hardening in the body of the gear beneath the gear teeth surfaces.
Generally, to accomplish hardening, the material to be hardened must be raised above a transformation temperature and then quickly cooled by quenching to induce hardening. Factors affecting the resulting hardness pattern include the depth to which the material is heated, the degree to which the heated temperature exceeds the transformation temperature and the rate of cooling.
Previous methods of hardening the toothed surface of a gear have included the use of induction heating the gear teeth followed by a quenching step. A circular inductor coil closely spaced from the undulating gear teeth surface generally exposes the radially outermost regions of the gear teeth to a greater degree of induction heating then the connecting regions between the gear teeth, with the result that the temperature and depth of heating is correspondingly greater at the outer region of the gear teeth. The differently heated regions will then be cooled at differing rates in the liquid quenching process, with the result that the hardness pattern developed thereby will be uneven across the gear teeth, with excessive hardening to depth beneath the gear teeth surfaces and with insufficient hardening at the connecting surfaces between the gear teeth. Accordingly, in order to successfully harden gear teeth by induction heating and liquid quenching it is necessary to heat the gear teeth to a preselected temperature uniformly to a controlled depth and then immediately quench so that the surfaces of the geared teeth are uniformly and quickly cooled. Methods and apparatuses for providing the uniform heating to a preselected temperature uniformly to a controlled depth are described in U.S. Pat. No. 4,675,488; U.S. Pat. No. 4,757,170 and U.S. Pat. No. 4,894,501 assigned to the assignee of the present invention and incorporated herein by reference. While substantial success in uniform heating has been achieved, a problem still exists in the uniform quench hardening of helical gears.
The problem of nonuniform quenching is especially a problem in high production rate hardening of gears. Thus, elongated helical gears which must be surface hardened in great numbers are processed in hardening stations which quickly and automatically apply heat inductively to the gear followed by application of quenching fluid. One such method and apparatus is described in U.S. Pat. No. 4,894,501. In such a system, a gear is placed upon a support structure and lowered through a hardening apparatus. An upper element in the apparatus inductively heats a circular band in the gear adjacent the inductive heating element as the gear moves downwardly by the inductive heating element. A quench element is positioned directly below the inductive heating element. The quench element sprays a quench fluid at the gear, quenching and hardening the toothed surface. The entire gear is hardened as it passes downwardly past the inductive heating element and the quenching element. In the hardening of large numbers of gears, such a station is automated and repetitively performs the hardening cycle on gears which are successively loaded upon the support element. The gears are raised, lowered and hardened and removed from the support element. The gears are often spun during the hardening operation so that a uniform heating and quenching pattern around the periphery of the gear is achieved.
Applicant has found that in the above described apparatus and method, quench fluid flow over the tooth surface of the gear may not be uniform. Irregularities in the fluid flow result in irregularities in the cooling rate of the heated surface and irregularities of the hardness pattern of the finished gear. In hardening wheel gears with the teeth on the outside surface of the gear, the problem of nonuniform quench is emphasized because the spinning wheel gear flings quench fluid from its surfaces by its spinning action. Fluid is flung first from the grooves between the teeth, flows out the sides of the teeth and is ejected from the tip area. For helical gears, achievement of uniformity of flow of quench fluid to the entire toothed surface is particularly difficult. The teeth of a helical gear are inclined with respect to the axis of the gear and the flanks of the gear teeth present surfaces at difficult angles for quenching. The achievement of high production rate, induction heating and quench hardening is thus limited by the ability to achieve a uniform quench of complex gear shapes, particularly in the case of helical gear shapes.